# Ray Optics - JEE Main Physics Preparation

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Published on February 15, 2014

Author: ednexa

Source: slideshare.net

Ray Optics Real and Virtual Images If light rays, after reflection or refraction, actually meets at a point then real image is formed and if they appears to meet virtual image is formed.

Reflection of Light When a ray of light after incidenting on a boundary separating two media comes back into the same media, then this phenomenon, is called reflection of light.

(1) ∠i = ∠r (2) After reflection, velocity, wave length and frequency of light remains same but intensity decreases. (3) There is a phase change of  if reflection takes place from denser medium.

Reflection from a Plane Surface Plane Mirror The image formed by a plane mirror is virtual, erect, laterally inverted, equal in size that of the object and at a distance equal to the distance of the object in front of the mirror. (1) Deviation (δ): Deviation produced by a plane mirror and by two inclined plane mirrors.

(2) Images by two inclined plane mirrors: When two plane mirrors are inclined to each other at an angle θ, then number of images (n) formed of an object which is kept between them. (i) o  360o   ; If 360  even int eger   n    1      (ii)

If 360o   odd integer then there are two possibilities (3) Other important informations (i) When the object moves with speed u towards (or away) from the plane mirror then image also moves towards (or away) with speed u. But relative speed of image w.r.t. object is 2u.

(ii) When mirror moves towards the stationary object with speed u, the image will move with speed 2u in same direction as that of mirror. (iii) A man of height h requires a mirror of length at least equal to h/2, to see his own complete image. (iv) To see complete wall behind himself a person requires a plane mirror of at least one third the height of wall. It

should be noted that person is standing in the middle of the room. Curved Mirror It is a part of a transparent hollow sphere whose one surface is polished.

Concave mirror converges the light rays and used as a shaving mirror, In search light, in cinema projector, in telescope, by E.N.T. specialists etc. Convex mirror diverges the light rays and used in road lamps, side mirror in vehicles etc. 1. Terminology (i) Pole (P): Mid point of the mirror (ii) Centre of curvature (C) : Centre of the sphere of which the mirror is a part. (iii) Radius of curvature (R): Distance between pole and centre of curvature.

(R = –ve , R concave convex = +ve , R plane = ∞) (iv) Principle axis: A line passing through P and C. (v) Focus (F) : An image point on principle axis for an object at ∞. (vi) Focal length (f) : Distance between P and F. (vii) Relation between f and R : (f concave = –ve , f convex f = + ve , f R 2 plane =∞ ) (viii) Power: The converging or diverging ability of mirror

(ix) Aperture: Effective diameter of light reflecting area. Intensity of image ∝ Area ∝ (Aperture)2 (x) Focal plane: A plane passing from focus and perpendicular to principle axis. 2. Sign conventions (i) All distances are measured from the pole. (ii) Distances measured in the direction of incident rays are taken as positive while

in the direction opposite of incident rays are taken negative. (iii) Distances above the principle axis are taken positive and below the principle axis are taken negative. Table 1: Useful sign Concave mirror Conve Real image (u ≥ Virtual x f) image (u< f) mirror

Distance of u → – u → object u → – v → + – Distance of f → – v → image v → – O→+ + Focal length f I →+ f → – R→ – + → Height of object m → + O → O →+ + Height of image I I + → – → Radius of R → curvature R → + – m → Magnification m + → –

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